The present invention relates generally to apparatus and methods for reducing print misregistrations in color printing, and, more specifically, to object-based choking and spreading procedures that include object combining operations for generation of color film separations.
Printing is well known. To print in color, a printing machine combines a plurality of color images from each of a plurality of platens. Each platen reproduces a portion of a final combined image from a plate pressed onto a recording medium. Photographic procedures create each plate from a film. A film for each plate typically corresponds to one primary color, with a set of films referred to as a set of film separations. FIG. 4 is an illustration of a set of film separations using cyan, magenta, yellow and black primary colors. The separations produce a plurality of plates for a printing machine. By varying particular color amounts contributed by each film separation to a final combined image, a large variety of colors for the combined image is possible. FIG. 5 is a schematic representation of one example of a printing press for color images. The printing press includes multiple platens, one for each separation layer as identified in FIG. 4, for example. Each roller adds its respective color, as identified by its plate created from its corresponding film, to a recording medium passing beneath the platen. Each subsequent platen contributes its color to the recording medium until all platens have contributed their respective colors to the final image. Each color's contribution to any particular part of the final image is variable from zero percent to one hundred percent.
The method requires precise synchronization and alignment of the platens to each other and to the advancing recording medium. Any positional or timing error in laying down the individual patterns of each color separation results in improper alignment of the various images. The printing industry refers to this improper alignment as print misregistration.
FIG. 6A is an illustration of a desired registration of a black text character "a" on a cyan background. If there is some error in recording of either the character "a" or the background, misregistration results. FIG. 6B is an illustration of a misregistration of the character "a" on its cyan background. The misregistration is particularly noticeable in that the misregistration exposes portions of the recording medium which were not inked. The fact that each color of the FIG. 6B image is a single ink (contribution from a single platen) makes misregistration more likely. To avoid this problem, printers long ago developed photographic techniques called choking and spreading to make misregistration less noticeable. Choking and spreading refers to controlling the images of the film separations to compensate for misregistration. Rather than requiring exact alignment of the respective images of the film separations, the film separation allows overlap of the various colors. Thereafter, small misalignments are not as noticeable.
Typically, in spreading and choking, the lighter color overlaps the darker color. Printers perceived that a dark area with a lighter color mixed in was less noticeable than either a white border around mismatched images, or darker colors in lighter images. Thus, a region bordering images of different colors which abut one another is actually made a third color.
Spreading refers to enlarging an object on top of its background, while choking refers to enlarging a background to overlap the object. These photographic techniques include actual enlargement of the subject image.
Pre-press processing systems generate the film separations for use in color printing. It is known to use computer-assisted, pre-press systems to facilitate generating film separations. The computer-assisted, pre-press systems electronically store the various object and background images. A pre-press system operator can electronically manipulate the various images to choke or spread a desired image onto another object. These pre-press systems typically store a page as a large rasterized image having an array of pixels. The rasterized image associates each pixel with four color bytes. Typical pixel densities range from about 50 dots per inch (DPI) to about 2000 DPI, with 300 DPI common. Thus, representing an ordinary notebook size page of 8.5".times.11" requires over 32 million bytes at a resolution of 300 DPI with four color bytes per pixel. Special encoding reduces the size of the raster, but as the number of pages increases, the total memory requirements become unwieldy. Image storage is a disadvantage with these systems. Spreading and choking is relatively easy with rasterized images because the system knows each image's pixel value and position. Changing particular images is straightforward.
Another disadvantage of the rasterized approach relates to editing and changing the images. Unless an unlimited amount of image storage capability is available, spreading with rasterized images is a one-way process. Reversing one or more spread steps is difficult unless the operator stores each intermediate result prior to executing a subsequent spreading step. For example, if a customer desires to change a font in a headline, the operator typically performs all the spreading, choking and editing operations again from a rasterized image with the new font.
Current page layout and illustration products, including QuarkXPress (available from Quark, Incorporated), Aldus Freehand (available from Aldus Corporation), and Adobe Illustrator (available from Adobe Systems Incorporated) for the Macintosh provide functionality to choke and spread ink separations in limited situations. These systems offer an advantage over prior art raster systems memory storage and editing limitations because they are object-based systems. The information stored to recreate a page is less than with the raster system, saving disk storage space. Additionally, the user can re-edit jobs without starting from an initial drawing and recreating all the chokes, spreads and other operations anew. The object-based systems output results in a page description language rather than as encoded rasters. Prior art systems have limited choking and spreading functionality dependent upon a particular page description language the system employs.
Object-based systems use a page description language to define objects on a rendered page of a display medium, such as a video display terminal (VDT) or a printed page. The page description language defines each object with a path which traces its outline. Rendering is the process of either filling a path with a fill color, stroking a path with a stroke color, or flowing text within an object's path.
"PostScript" is a commonly used page description language. Adobe Systems, Inc. created and promulgates the PostScript language. PostScript functionality and definitions are defined in "PostScript Language Reference Manual" Addison-Wesley Publishing Co., Inc., 1985, hereby incorporated by reference for all purposes.
Existing systems typically depend upon four page-description-language-related features to perform spreading and choking. These four operations include fill (and eofill), stroke, overprint and clip (and eoclip). The fill operator paints an interior of an object a specified color. The stroke operator produces a variable width line of specified color centered on a stroked object's outline. The overprint operator is used to generate ink separations. Overprint allows addition of color to selected separations when drawing an object without altering other separations. The alternative is knockout printing, which automatically overwrites all non-painted separations with white. The clip operator limits an extent of a rendering region on a page.
A simple prior art solution to spreading is to render a stroke of a spreading object using overprint to overlap a particular color into an abutting color. Unless the spreading object lies completely within the interior of the choked object, this method will produce distorted spread objects as the spread object will extend into inappropriate areas. A next level of spreading complexity uses the clipping operator to limit all renderings to an area lying within the object receiving the spread. Subsequent stroke overprinting only appears within an area of overlap, that is, where the stroked object falls within the object receiving the spread.
This improved prior art spreading mechanism has limitations in at least four situations. A user faced with one of these situations must either alter the design, or employ the raster-type system.
1. A user cannot stroke an object and subsequently spread the stroke into an abutting object's fill or stroke. Only an object's fill can spread into another object's fill. Current page description languages do not permit a stroke to follow the outline of the stroke of an object. PA1 2. Obstructors, additional objects lying on top of the spreading object, prevent Spreading because the outline of the spreading object no longer defines the boundary to be spread across. PA1 3. Some page description languages do not have an overprint feature, preventing formation of overlapping colors from different objects. PA1 4. Even if none of the previous three limitations are present, the prior art method will provide an incorrect result in certain cases when the spreading object lies under receiving object. If one or more edges of the spreading object lie exactly under edges of the receiving object, then stroking either the spreading object or the receiving object produces the wrong result. This is because where their outlines overlap, neither outline separates the two colors, but rather borders the background.